1. Technical Field
The invention is related to semiconductor imaging devices and in particular to such a device that can be fabricated using a standard CMOS process. The present invention provides a method and an apparatus to providing blooming protection for a pixel in an array while extending dynamic range.
2. Background Art
A complementary metal-oxide semiconductor (CMOS) image sensor is a device for converting optical images to electrical signals. That is, it responds to the visible light, and the signal electrons thus formed are converted to voltages. Then the voltages are subjected to a signal processing to reconvert the voltages to image information.
To provide context for the invention, an exemplary CMOS imaging system is described below. FIG. 1 is a block diagram of an imaging system 100 including an image capture circuit 110, a control circuit 112, an image processor 114 and an output device 116 all intercoupled as depicted.
Image capture circuit 110 includes an array of photoactive pixel circuits whose surfaces receive light projected from an image. The light energizes the pixel circuits to produce pixel signals as a function of the light energy received. Decoding circuits select among the pixel circuits to produce an analog image signal VOUT at a node 111 that is representative of the captured image.
Control circuit 112 has an input 118 for receiving a user initiated control signal VCONTROL and an output bus 113 that provides digital address data for selecting pixels. A digital initialization signal reset is produced at a node 117 prior to capturing an image in order to clear the array of residual signals from a previous capture. Control signal VCONTROL allows a user to control exposure time, the amount of zoom, or to provide other imaging features.
Imaging system 100 may be utilized in a wide variety of devices including copiers, scanners, cameras, medical devices, toys, machine vision systems, vehicle navigation systems, video telephones, computer input devices, surveillance systems, auto focus systems, star trackers, motion detection systems, image stabilization systems, data compression systems for high-definition television or other imaging devices. Such devices are generally driven with a low voltage, and a single chip is sufficient in most cases.
In order to meet the increasing need for high speed image sensor devices, it is becoming necessary to integrate the image sensor array with other digital circuitry that controls the operation of the array and processes the array output. Integration of the image sensors with CMOS support circuitry is desirable because of the low power consumption characteristics, maturity and common availability of CMOS technology.
A CMOS imaging circuit may be composed of an array of pixels. FIG. 2 is a block diagram of one pixel cell 210 of an array of photoactive pixel circuits. Each cell 210 includes a photogate 212, a charge transfer section 214 adjacent the photogate 212 and a readout circuit 216 adjacent the charge transfer section 214. FIG. 3 shows an array of many photoactive pixel circuits cells 310 formed on a silicon substrate 320.
A conventional CMOS imager pixel typically employs a phototransistor or photodiode as a light detecting element, and is usually operated as follows. First, the pixel photodiode is reset with a reset voltage. This removes electrons from the “charge well” or “pixel well” of the photodetector, thereby placing an electronic charge across the capacitance associated with the photodiode. Next, the reset voltage is removed and the photodiode exposed to illumination. The incoming light creates free electrons in the pixel well, causing the charge stored across the photodiode capacitance to decrease at a rate proportional to the incident illumination intensity. FIG. 4 shows a schematic view of a pixel well as light shines upon it. At the end of an exposure period, the change in diode capacitance charge is detected and the photodiode is reset. The difference between the reset voltage and the voltage corresponding to the final capacitance charge indicates the amount of light received by the photodiode.
Conventionally, CMOS image sensors have several limitations, for example, limited dynamic range and blooming. Both are discussed below.
A problem from which conventional CMOS image sensors suffer is a phenomenon called blooming. Image sensor devices that integrate charge created by incident photons have dynamic range limited by the amount of charge that can be collected and held in a given photosite. For example, the maximum amount of charge that can be collected and detected in a pixel is proportional to the pixel area. As discussed above, during the optical integration period, electrons are created in a pixel well at a rate proportional to the light intensity reaching the sensing area. As the electrons are collected in the photodetector, it begins to fill. If the photodetector charge well becomes full of charge, it becomes saturated and blooming may occur.
Blooming is a phenomenon in which excess charge from a pixel spills over into adjacent pixels, causing blurring and related image artifacts. Blooming may cause the neighboring pixels to look brighter than an accurate representation of the light absorbed by the photodiode in that pixel. This phenomena is illustrated in FIG. 4, which shows electrons escaping from a full pixel well to a neighboring well.
One solution for blooming is shunting off the excess current caused by the incoming light once the pixel becomes full. A mechanism for doing this uses the reset transistor, which is ordinarily used to remove all electrons from the pixel well before beginning an exposure. During exposure, the reset transistor can be biased slightly to operate in the sub-threshold region, allowing excess charge to flow to the reset drain, thereby acting as an anti-blooming drain.
Another problem conventional CMOS image sensors have is limited dynamic range. CMOS imagers generally are characterized by a linear voltage-to-light response, that is, the imager output voltage is approximately linearly proportional to the integrated intensity of the light incident on the imager. The imager output voltage can be characterized by a dynamic range, given as the ratio of the maximum detectable illumination intensity of the imager to the minimum detectable illumination intensity of the imager. It is well understood that the dynamic range of the output voltage sets the overall dynamic range of the imager. The illumination intensity that causes the photodiode capacitance charge to be completely dissipated prior to the end of the exposure period, thereby saturating the pixel, sets the upper end of the pixel dynamic range, while thermally generated photodiode charge and other noise factors set the lower end of the pixel dynamic range. If the dynamic range of a scene to be imaged exceeds the dynamic range of an imager, portions of the scene will saturate the imager and appear either completely black or completely white. This can be problematic for imaging large dynamic range scenes, such as outdoor scenes.
The graphs illustrated in FIG. 5 show an output voltage of a pixel (Bus) and an output voltage of a photo-diode voltage (Vph) within the pixel. Note that, as shown, this pixel is configured to have an output voltage inversely proportional to the number of electrons collected in the photodiode. The photo-diode voltage (Vph) starts clamping near the end of the integration period, indicating that the pixel well is full, and the excess charge is leaking to the substrate. The pixel output voltage (Bus) clamps earlier than the photodiode voltage because of the voltage drops of the junctions in the pixel structure. This means that even before the pixel is completely full, the output of the pixel loses gain and the dynamic range of the pixel circuit is limited. Note that when the output voltage clamps, the pixel no longer has a linear voltage-to-light response; the output clamps at approximately the maximum linear output of the pixel. (In this pixel, because the pixel output voltage falls with increasing numbers of collected electrons, the “maximum linear output” is a low or a near-minimum voltage. In alternative pixels, the “maximum linear output” may be a high or a near-maximum voltage.) The problems of blooming and limited dynamic range are related. The use of a reset transistor to prevent blooming is very sensitive to the voltage applied to the reset transistor gate. If the reset voltage is too low, no electrons (or an insufficient number of electrons) will be shunted out of the pixel well and blooming may occur. If the reset voltage is too high, blooming will be prevented, but at the cost of limiting the dynamic range of the pixel, because electrons will be shunted out of the pixel well before the well is full, limiting the maximum charge that can be collected.
The proper voltage to be applied to the reset transistor depends on a variety of factors, including manufacturing process parameters and the temperature of the imager array. Thus, the proper bias voltage varies from imager array to imager array, and varies over time as operating conditions change. Prior approaches to biasing the reset transistor that used a fixed bias voltage failed to account for these variations, and suffered from blooming or reduced dynamic range.
It is, therefore, desirable to take into account manufacturing process parameters, temperature dependencies and other factors that might affect the transistor operation in providing a method and an apparatus to determine the voltage level needed to be applied to a reset transistor gate in a real pixel to prevent blooming.